1. Field of the Invention
The present invention relates to desiccant air dryers and, more particularly to a system and method for controlling the regeneration cycle of a twin tower desiccant air dryer.
2. Description of the Related Art
Air dryers for railway use are typically a “pressure-swing adsorption” type, also referred to as a twin-tower, desiccant air dryer. The basic control scheme for switching between the two columns of desiccant is a fixed timer enabled by a “compressor ON” signal from the compressor controls. Whenever the compressor is running, the air dryer cycles between two columns of desiccant at a fixed time cycle to direct wet product air through one column to remove the water vapor, thus resulting in dry product air, while simultaneously taking a fraction of the dry product air and counter-flowing it through the other previously saturated column of desiccant to remove accumulated moisture. Although simple and robust, this control scheme is inefficient and wastes considerable energy.
A typical AAR locomotive air supply system consists of a compressor and two main reservoirs in series, MR1 and MR2. The air dryer is usually installed between MR1 and MR2, so that dry air is delivered to MR2. MR2 is used as an exclusive air source for the train braking system and is protected by a back-flow check valve between MR1 and MR2. The air in MR1 is used for other locomotive air consumers like the windshield wipers, horn, sanders, etc. When the air is consumed from either MR1 or MR2, the compressor will operate to recharge the system. If the air pressure in MR1 is less than MR2, the compressor will operate so that air flows into MR1 to recharge it, but air will not flow into MR2 until the pressure in MR1 is greater than the pressure in MR2. In this situation, the air dryer regeneration cycle is enabled by a compressor ‘on’ signal. Because there is no air flow between MR1 and MR2, however, there is no air flow through the air dryer. As a result, the dry product purge air consumed by the air dryer regeneration cycle is wasted.
The second inherent inefficiency of the existing fixed timer regeneration control scheme is that it assumes that the water content of the incoming “wet” air is constant, and the fixed timing cycle is based on the worst case for maximum flow and maximum wet air. The amount of water vapor in air is directly proportional to the saturation water vapor partial pressure, which has a highly non-linear, exponential-like, relation with temperature. For example, the saturation water vapor partial pressure at 0° F. is 0.01857 pounds per square inch absolute (psia); at 70° F. it is 0.3633 psia; at 125° F. it is 1.9447 psia, and at 150° F. it is 3.7228 psia. By contrast, air at 125° F. can contain 5.35 times as much water vapor as air at 70° F., and air at 150° F. can contain 10.2 times as much water vapor as air at 70° F. Thus, air at 125° F. can contain 105 times as much water vapor as air at 0° F., and air at 150° F. can contain 200 times as much water vapor as air at 0° F.
Thus, it is clear that an air dryer fixed cycle time regeneration cycle which is established on the water holding capacity of the desiccant bed and the water content of saturated air at the maximum inlet air temperature, e.g., 150° F. will cycle much more frequently than is necessary for lower temperatures and thus will waste dry product purge air. For example, a system designed to handle 150° F. saturated air, will be cycling 10.2 times too much at 70° F. and 200 times too much at 0° F. Thus, at 70° F., there is an opportunity to save approximately (17%−17%/10.2)=15% product air and compressor energy this is being wasted. As a result, there is a need in the art for an air dryer having a more efficient regeneration cycle control system.